fertilisation

Question 1

How do sperms gain their fertilising capacity?

Answer 1

1. sperm capacitation
2. interaction with the female reproductive tract

Question 2

Sperm capacitation

Answer 2

• further and final maturation process in which sperms: happens in the female tract as the sperm get in.
  1. change to a state of hyperactivated
  motility.
  2. gain ability to bind to the oocyte’s zona pellucida and afterwards undergo the acrosome reaction.
  3. gain the capacity to fuse with the oocyte.

Question 3

female tract interaction mechanisms of sperm

Answer 3

-Chemotaxis
-Thermotaxis
-Rheotaxis
-Boundary-following navigation

Question 4

Chemotaxis

Answer 4

— Chemical guiding mechanism.
— Sperm responding to a gradient of chemoattractant e.g. steroid hormones.
— Molecular and behavioural mechanisms yet to be fully understood.

Question 5

Thermotaxis

Answer 5

— Temperature guiding mechanism.
— Female tract consists of different areas with marginal differences in temperature.
— Sperm responding to changes in the extracellular temperature gradient.

Question 6

Rheotaxis

Answer 6

— Ability to respond to fluid currents in the female tract environment.

Question 7

Boundry- following navigation

Answer 7

— Ability to turn corners in response to surface boundaries
— Human sperm with preference to follow boundaries on the left or right hand side have
been shown to possess higher DNA integrity than straight swimming sperm (Huang et

Question 8

What modulates the female tract interaction of sperms?

Answer 8

CatSper channels

Question 9

What are CatSper?

Answer 9

• cation channel in sperms
• located in the flagella region
• responsible for calcium influx (calcium influx happens faster rate at body temp than room temp)
• responds to diverse stumuli : Em, pH, progesterone, prostaglandins, small organic molecules

Question 10

What is the main change that occurs in sperm capacitation?

Answer 10

loss of cholesterol from cell membrane of sperm cells
=> Labs culture:
if you supplement culture media with substances that can take cholesterol away you see sperms switch to hyperactivity, which is a obvious sign they are undergoing cell capacitation.
- so female tract has cholesterol acceptors that takes away cholesterol from the sperm
- cholesterol leaving alters PL ratio in the cell membrane and also as a result affects the availability of soluble proteins in the membrane like soluble adenine cyclase become more available due to cholesterol loss further facilitating fertilisation.

Question 11

What happens when progesterone(P4) binds to its receptors on cell membrane?

Answer 11

-P4 binds to ABHD2 receptor and stimulates CatSPer channels
-influx of Calcium allows for motility of sperm

Question 12

What happens after capacitation?

Answer 12

• Once a few capacitated sperm make it to the site of fertilisation, they come in contact with the cumulus-oocyte complex (COC) and a much higher concentration of follicular fluid (FF).
• FF in addition to secretions from the COC e.g. progesterone further modulates
  the spermatozoa to initiate the process of sperm-oocyte interaction and subsequent fertilization.

Question 13

What are the events in Sperm- oocyte interaction?

Answer 13

Penetration of the cumulus oophorus
Sperm-zona binding
Acrosome reaction
Sperm-oocyte fusion and oocyte activation

Question 14

2. Penetration of the cumulus-oophorus

Answer 14

Once sperm come in contact with the
cumulus-oophorus cell mass surrounding the
oocyte, the enzyme hyaluronidase present on
the surface of the sperm head dissolves
hyaluronic acid (hyaluronan).

Question 15

2. Sperm-zona binding

Answer 15

ZP are located between the theca and granulosa in the oocyte , once the sperm penetrates through the cumulus it enters the oocyte :

• Four zona pellucida (ZP) glycoproteins
  expressed – ZP1, ZP2, ZP3, ZP4.
• Studies across mammalian species have
  shown that sperm bind to ZP2 and ZP3
  via receptors present on the sperm’s
  plasma membrane.
• Recent studies by Yauger et al. (2011)
  and Baibakov et al. (2012) show that
  human sperm bind to ZP2.
  => transgenic mice used

Question 16

2. Acrosome Reaction

Answer 16

• Triggered in response to sperm-oocyte
  interaction
• Fusion of the sperm’s plasma membrane
  with it’s outer acrosomal membrane.
• Results in the release of hydrolytic and
  proteolytic enzymes e.g. hyaluronidase
  and acrosin.
  => the sperms that survive and make it through to the oocyte are the ones that undergo acrosome reaction, but some do undergo acrosomal reaction randomly, but the ones that succeed are the ones that are in the right place at the right time.

Question 17

3. gain capacity to fuse with Oocyte : Sperm-oocyte fusion & oocyte activation

Answer 17

• Juno-Izumo binding.
• Intracellular calcium spike
• Cortical reaction
• Release from meiotic arrest and completion of meiosis II.
• Incorporation of sperm DNA and pronuclear formation.
• Embryo cleavage

Question 18

What is Juno?

Answer 18

• juno is the egg Izumo receptor and is essential for mammanlian fertilisatuon

Question 19

3. gain capacity to fuse with Oocyte : activation

Answer 19

• Phospholipase C Zeta (PLC ζ) sperm introduces this and widely accepted as the oocyte activation factor introduced by sperm.
• phospholipase C zeta activates inositol trisphosphate (IP3) production, leading to repetitive calcium release (oscillations) from the oocyte’s calcium stores.
• Has been identified in all species studied so far, including human

Question 20

when looking at fertilisation the two key morphological features are :

Answer 20

presence of pronuclei = 2 pronuclei
presence of polar body = 2 polar body showing meiosis 2 is complete
identification of these two factors helps identify if fertilisation took place correctly
if there are <2 or >2 pronuclei of polar bodies something went wrong

Question 21

pro-nuclei

Answer 21

2PN - normal fertilisation.
Normally fertilised oocytes should contain two juxtaposed and centrally located PN, with distinctly clear membranes and nucleolar precursor bodies
(NPBs).

0PN - No fertilisation.

1PN - Abnormal fertilization. Contains a haploid
set of chromosomes, from only one parent.
There are cases where a 1PN zygote contains a diploid set of chromosomes, resulting from errors in the
synchrony of PN formation/fusion. Zygotes
in this category often have 2 polar bodies.

≥3PN -Abnormal fertilisation. Contains additional set(s) of chromosomes, which could be either digynic or diandric. Significantly linked
to the formation of aneuploid embryos.

Question 22

polar bodies

Answer 22

0PB- Oocyte at the Metaphase I
stage of meiosis.

1PB- Mature oocyte at the Metaphase II stage of meiosis with one polar body extruded.
No fertilisation.

2PB- Oocyte has completed second meiotic division and extruded second polar body in response
to fertilization.

Question 23

fertilisation and embryo development

Answer 23

1. 8 cell
2. morula = cells tightly packed, smaller gap junction , the precursor of early blastocyst formation
3. early blastocysts
4. expanded blastocysts
5. hatching blastocysts
6. hatched blastocysts

=> when blastocysts impants in the uterus depends on rate of development